There is no autonomous mode for these APs. EWC is the correct configuration method.
Ensure there are no wireless controllers reachable within the network from the EWC AP (Local Subnet, DHCP Option 43, DNS). During the boot process, if the AP discovers a controller in the network, it will join the controller and will not proceed with the EWC controller script.
Follow the link below and make sure to review the pre-requisites.
https://www.cisco.com/c/en/us/products/collateral/wireless/embedded-wireless-controller-catalyst-access-points/white-paper-c11-743398.html
Also, share the complete boot log for this AP to analyze the issue.
Jagan Chowdam
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